Which project does this relate to?

Router

Describe the bug

Hi, I think the type is broken for the useServerFn.
I've reproduced and noticed that if I need to use the onSuccess callback I loose the types

Your Example Website or App

https://github.com/Christopher96u/ts-issue-demo

Steps to Reproduce the Bug or Issue

1. Go to the index/home route

Expected behavior

I expect as user, to have the onSuccess types back or improve the docs if this is the right behaviour

Screenshots or Videos

If I comment out the onSuccess callback there is no TS issues

If I leave the onSuccess callback there is a TS issue

If we use the useServerFn hook at top level (not inside the useMutation hook, we get the types back

Platform

• Router / Start Version: [e.g. 1.121.0]
• OS: [e.g. macOS, Windows, Linux]
• Browser: [e.g. Chrome, Safari, Firefox]
• Browser Version: [e.g. 91.1]
• Bundler: [e.g. vite]
• Bundler Version: [e.g. 7.0.0]

Additional context

No response